This invention pertains generally to mapping systems using a synthetic aperture in an aircraft and particularly to any system of such type wherein the synthetic aperture radar is operated following so-called focused synthetic aperture techniques.
It is known that systems including synthetic aperture radar carried in an aircraft (such as the system described in the article entitled "Performance of a Synthetic Aperture Mapping Radar System," by J. A. Develet, Jr., IEEE Transactions on Aerospace and Navigational Electronics, September 1964, pp. 173-179) may be used to generate a radar map having a resolution equivalent to the resolution of a photograph. Briefly, the system described in the just-cited article is arranged first to record, on photographic film, images representative of the time history of the echo signals from a swath of the terrain underlying an aircraft carrying a synthetic aperture radar and then, using an optical signal processor, to process the images on the photographic film in order to generate the desired radar map of the swath of the underlying terrain.
Although the just-outlined system for generating a radar map is adequate in many operational situations, there are various tactical situations encountered by the military (such as situations involving weapon delivery or damage assessment) wherein a system of such type is unsatisfactory. The relatively long processing time inherent in the use of photographic equipment is intolerable in such tactical situations wherein decisions must be made as soon as information is available.
In known systems designed to reduce processing time to a minimum so that a radar map may be generated in "real-time" various well known digital processing techniques have been adapted to the problem of converting echo signals from a synthetic aperture radar to a radar map for display on a viewing screen in a device such as a cathode ray tube. With any digital processing technique adapted to produce a radar map in "real-time" on board an aircraft, practical considerations, such as weight, size and complexity, are limitations on the amount of digital processing equipment dedicated to the function of map making. On the other hand, however, if a radar map is to be generated in "real-time" from radar echo signals out of any known synthetic aperture radar, the amount of information which must be processed to generate a radar map having satisfactory resolution is extremely great. The capacity of the digital processor used must, perforce, be correspondingly great, especially if the resolution possible with a focused synthetic aperture radar is to be attained.
The capacity of a digital processor used to process information for a radar map must, according to the prior art, be greater when synthetic aperture radar is operated in a "squinted" mode rather than a "broadside" mode. In the latter mode, the centerline of the beam of the synthetic aperture radar is maintained in the vertical plane orthogonal to the course line of the aircraft carrying such radar; in the former mode, the centerline of such beam is maintained in a vertical plane to which such course line is inclined at an acute angle, say in the order of 45.degree.. The orientation of the beam in the broadside mode of operation results, for almost all practical cases, in the reduction of the effects of Doppler acceleration due to relative motion between the aircraft and any selected point within the area being mapped to an insignificant degree. On the other hand, however, the effects of Doppler acceleration due to relative motion between the aircraft and any selected point within the area being mapped cannot be ignored in the squint mode if an appreciable area is to be mapped.